Helo, Y; (2015) Cerenkov emission in radiotherapy. Doctoral thesis , UCL (University College London).

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Abstract

A new potential quality assurance (QA) method is explored for clinical electron beams and clinical proton beams based on imaging and measuring Cerenkov light. A simulation was performed of the deposited energy and of Cerenkov production in water using Geant4. Monte Carlo simulation was used to predict the measured light distribution around the water phantom, to reproduce Cerenkov images and to find the relation between deposited energy and Cerenkov production. The camera was modelled as a pinhole camera in Geant4, to attempt to reproduce Cerenkov images. The potential of using a standard commercial camera to image Cerenkov light generated from electrons in water for fast QA measurement of a clinical electron beam was explored and compared to ionization chamber measurements. The new method was found to be linear with dose and independent of dose rate (to within 3%). The uncorrected practical range measured using Cerenkov images was found to overestimate the actual value by 3 mm in the worst case. The field size measurements underestimated the field sizes at the edges by 5% without applying any correction factor. Still, the measured field size could be used to monitor relative changes in the beam profile. Finally, the beam-direction profile measurements were independent of the field size within 2%. We found that imaging Cerenkov emission from a breast phantom during electron irradiation could be a suitable tool to monitor the dose and the dose rate consistency with high precision and short-term repeatability better than 3% except when measuring very low doses. Cerenkov light measurements were linear with dose and independent of dose rate. The maximum light intensity occurred at an angle of 45.0°. We were unable to identify the regions of the phantom with higher scattering and absorption properties, designed to mimic diseased tissues using images of Cerenkov emission of an optical breast phantom. We found that the Cerenkov light emissions in proton therapy can be divided into two distinct mechanisms: a fast component due to prompt gamma interactions (99.13%) and neutron interactions (0.87%), and a slow component due to radioactive decay. The simulated depth distribution of the Cerenkov emission shows a strong relation with the depth distribution of the induced radioactive isotopes, which emit positrons. The fast component was found to be linear with dose and independent of dose rate, while the slow component increases non- linearly with dose and is highly dependent on dose rate. Imaging Cerenkov light during electron radiotherapy or proton therapy could be used as a very quick routine QA tool.

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